This Couldn't Happen! III

The Red Sox and Yankees were playing a game at Yankee Stadium. It was all tied up in the top of the ninth inning 5-5. David Ortiz stepped up to the plate and hit a walk-off home run. The Red Sox won the game!

Why couldn't this happen?

Hint

A walk-off home run is a home run that ends the game because there is no way for the other team to comeback.Hide

This is highly pedantic, but there's a way it could happen. If the Yankees last game in Boston for the year got rained out, and the teams still had a scheduled game in New York, the game might be made up as part of a double header in New York. Thus there could be a game at Yankee stadium where the Red Sox are the home team.